Can't get the speaker to stay on

Discussion in 'The Projects Forum' started by m_to_the_b, Jul 4, 2009.

  1. m_to_the_b

    Thread Starter New Member

    Jun 24, 2009
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    0
    Hi all. Am setting up a simple light activated alarm circuit, as shown below:

    [​IMG]

    The basic idea is this. When red laser light falls on the LDR (R1), current flows into the base of the NPN transistor, allowing flow through the relay (Sw). This allows the upper flow to connect, and the loop in the relay keeps it on. While flowing through this loop, the current sounds the speaker. This relay latch means that once the speaker is on, it doesn't need the laser anymore.

    So, the hope is that if a laser hits the LDR, the speaker will switch on, and stay on. It'll have to be turned off at the battery.

    If I replace the speaker with an LED, this works fine. Laser hits LDR, LED turns on, light stays on, even when the laser is taken away.

    But once I put the speaker back, it will turn on, but it won't stay on. It sounds while the laser is on the LDR, but then turns off as soon as the laser moves. It's driving me crazy. The speaker is a 12V piezo, and I've tried a smaller one (3-15V). I figure the 9V battery doesn't have the guts to run through the relay (500Ω) and keep the speaker on. Do I need a bigger battery? I wired up two 9Vs in series, and it didn't help.

    Any suggestions of circuit alterations, or possible new/replacement components would be greatly appreciated.

    Thanks!
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    What appears to be missing from the circuit is any latching function. Is the relay a latching relay by any chance?

    Nevermind. I see that your relay is a reed-switch.

    hgmjr
     
  3. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    If the latch were working as you intended it to, what means of removing the drive from the speaker were you relying on?

    hgmjr
     
  4. m_to_the_b

    Thread Starter New Member

    Jun 24, 2009
    6
    0
    How do I turn off the switch once I'm finished? I just unplug the battery. I guess I'll eventually install a switch right near the battery.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A piezo speaker (transducer) makes a single "click" when DC is applied to it. It needs an AC signal to drive it.
    A piezo beeper has a built-in oscillator so it beeps continuously when DC is applied to it.
    I hope you have a beeper, not a speaker.

    Your link to your schematic doesn't work so we do not know how you wired the relay.
     
  6. m_to_the_b

    Thread Starter New Member

    Jun 24, 2009
    6
    0
    Apologies - I hope this picture of the circuit comes through.

    It's a beeper, by the way - DC sets it off.

    [​IMG]
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    The current of the beeper is not high enough to hold the relay turned on. Then the relay coil does not receive enough voltage.
     
  8. millwood

    Guest

    two problems with a piezo:

    1) it has very high resistance so that there may not be enough current going through the piezo (and the coil of the reply) to keep relay closed once the light is removed.

    2) some piezos are not continuously conducting. they go through a rapid on/off/on/... sequence. when they go off, there is no current going through the coil and the relay goes open, shutting off the piezo.

    what you need is something of low resistance (to keep the coil on all the time), and is continuously conducting, and makes sound.
     
  9. m_to_the_b

    Thread Starter New Member

    Jun 24, 2009
    6
    0
    !! I think you've got it with the alternating issue !!

    But what can I use as a continuous, low resistance alternative to a piezo buzzer? Anyone got any ideas?
     
  10. millwood

    Guest

    one idea to use the existing setup is to use the relay to also turn on a diode which shines light on the photo sensor.

    it would work regardless of what piezo you are using.
     
  11. m_to_the_b

    Thread Starter New Member

    Jun 24, 2009
    6
    0
    Kick ass guys. The alternation of the Piezo was the problem. All I had to do was put a parallel LED in the relay loop, and that let enough current flow through when the piezo was alternating in the off position.

    [​IMG]
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    896
    You correctly show a PNP transistor but list the part number of an NPN transistor.
     
  13. m_to_the_b

    Thread Starter New Member

    Jun 24, 2009
    6
    0
    Apologies, that's an NPN. I drew the diagram wrong.
     
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